1. Field of the Invention
The invention relates to wireless communications. In particular, the invention relates to transport block size (TBS) signaling in wireless communications.
2. Background Information
In systems that include mobile devices, packets are generally sent using one of many modulation schemes. All packets sent during a given transfer generally conform to the same transport block size (TBS). The sender of the packets sends information telling the receiver the modulation scheme and transport block size, among other things, before the packets are sent. Therefore, once the packets are sent, the receiver knows the modulation scheme, the number of multicodes, and the transport block size of the packets and, therefore, can correctly decipher and receive the information.
Proposals have been made about increasing the number of bits allocated for the definition of the transport block size (currently 6 bits) to allow for a higher resolution. These proposals have been particularly common with those involved in Third Generation Partnership Project (3GPP) standardization work. Specifically, those individuals working in areas related to Wideband Code Division Multiple Access (WCDMA) and High Speed Downlink Packet Access (HSDPA). In HSDPA, the TBS field is placed, along with other information, in the high-speed shared control channel (HS-SCCH). FIG. 1 shows a diagram of an example allocation of HS-SCCH signaling bits.
In Release 5 of the 3GPP specifications, the TBS is signaled on HS-SCCH using a 6 bit TBS field. The meaning of these bits is tied to the modulation scheme as well as the number of channelisation codes. See 3GPP TS 25.321 v5.2.0 (2002-09), which is hereby incorporated by reference in its entirety, particularly Section 9.2.3 directed to signaling of Transport Block Size.
The transport block size (TBS) has to be the same for the first transmission and the retransmissions, since the information content has to be the same. The retransmissions can use different redundancy version (RV), i.e., different coding, different number of channelisation codes (small changes currently allowed). The TBS is signalled for each retransmission on the HS-SCCH along with the modulation scheme, channelisation code set, redundancy version and other HARQ parameters.
There is some overlap in the TBS between different modulation schemes and theoretically the modulation scheme can be changed in some special cases, but in practice it is not possible to change the modulation for the retransmissions. This can be a problem if the first transmission is done with 16QAM and, due to bad channel characteristics, QPSK would be better for retransmissions.
The problem has not yet been adequately solved. In one 3GPP proposal, a 7 bit TBS field was proposed for QPSK and a 6 bit TBS field was proposed for 16QAM. See 3GPP Tdoc R1-02-1045.
Another proposal is a mapping of the transport block sizes through a logarithmic conversion to minimize the worst-case relative padding in case the MAC-PDU is not of the exactly same size as the available transport block sizes. This is discussed in document R2-0221668, entitled “Signaling of Transport Block Sizes for HS-DSCH” (Ericsson). However, this approach assumes a fixed number of bits for the signaling of the transport block size, and the transport block sizes are dependent on the modulation and multicode setting. Therefore, in cases where a packet is transferred and an error has occurred and retransmission is requested, the retransmission according to this approach must occur using the same modulation scheme. Thus, a transmitting device (e.g., base station) cannot freely select a modulation scheme whenever a retransmission is required.
It is also possible that, since the TBS is always the same, there is no need to signal it always with the retransmissions (provided that the UE has received the HS-SCCH correctly for the first transmission) and the TBS could be ignored by the UE for the retransmissions. Thus, the Node B could change the modulation scheme and the number of channelisation codes freely for the retransmission and this type of operation mode could be switched on/off by higher layer signaling. The problem with this proposal is that if the UE does not receive the first transmission correctly, it does not get the TBS at all (since it may be nonsensical in the retransmissions if the Node B has changed the modulation or the number of channelisation codes) and the TB is lost.